The present invention is directed to a novel system for producing selectable color bands from a single emission source in a pulsed arc discharge lamp. More particularly, this invention is directed to a small emission source for a pulsed arc discharge lamp, consisting of a gas discharge means and a method of activating this discharge means.
The applications for a multi-color pulsed arc discharge system are very widespread, ranging from signal and warning lighting to color projection of graphic information. The ability to select any of several narrow color bands from a single source greatly enhances the multi-color options that can be employed in signal and projection applications.
Conventional (i.e.. non-pulsed) metal halide arc lamps typically comprise a fused silica tube with two electrodes, a rare gas for starting, a charge of mercury, and one or more metal halides, generally iodides. In operation, a starting voltage of about 300V is applied across the electrode gap causing the contents of the arc tube to vaporize, resulting in a high temperature, high pressure, wall stabilized arc in a gas, consisting principally of mercury vapor, ionized metal atoms and iodine molecules. The output spectrum (i.e., the color of the discharge) consists predominantly of the spectrum of the added metals. Color output for such lamps is tailored by varying the metal halides added to the arc tube. See for example, Waymouth, "Electric Discharge Lamps," Chapter 8, MIT Press, (1971).
In the case of pulsed arc discharge lamps, the prevailing wisdom generated through years of experience with conventional metal halide arc discharge lamps fails to provide even a hint as to what color can or will be obtained based upon a given selected metal halide. The pulsed arc discharge lamp operates at a much lower temperature than the conventional metal halide discharge lamp, and thus, all of the conventional theories regarding color generation are useless as predictors of success.
Low pressure sodium lamps have previously been suggested as light sources in photocopying applications, see for example Hug, U.S. Pat. No. 3,914,649. An example of pulsed high pressure sodium vapor lamps is found in Osteen, U.S. Pat. No. 4,137,484.